The present invention relates in general to microelectromechanical (MEM) devices, and in particular to a method for electrically addressing an array of MEM devices such as an array of MEM micromirrors or MEM memory elements to latch selected MEM devices in an actuated state.
Arrays of microelectromechanical (MEM) devices can be used for redirecting or switching light beams and for forming optical or mechanical memories for storing information. Surface micromaching based on conventional semiconductor integrated circuit (IC) processing technology allows such arrays of MEM devices to be formed integrally on a substrate without the need for piece part assembly. Many different designs of MEM micromirrors have been disclosed that can be used in such an array (see e.g. U.S. Pat. Nos. 5,867,302; 6,025,951; 6,198,180 and 6,220,561). With present addressing schemes, each MEM micromirror to be latched must be individually actuated so that a large number of electrical connections and attendant electronic circuitry are required for the operation of a MEM micromirror array. For example, an array of mxc3x97n MEM micromirrors, where m and n are each integer numbers, currently requires m times n electrical connections since each MEM device in the array must be operated and addressed independently so that it can be latched. What is needed is a way to simplify the number of electrical connections for addressing a large array of MEM micromirrors or other types of MEM devices which are to be formed as arrays. The present invention provides a solution to this problem by providing a method for addressing an array of mxc3x97n MEM micromirrors that requires a minimum of m+n electrical connections, thereby greatly simplifying the number of electrical connections and attendant electronic circuitry. The present invention is also useful for electrically addressing an array of MEM memory elements and any other type of MEM device which is formed as an array that must be electrically addressed for activation or readout.
The present invention relates to a method for electrically addressing an array of microelectromechanical (MEM) devices which can comprise, for example, micromirrors or memory elements or both. The method of the present invention comprises steps for switching all of the MEM devices in a column of the array from a first state to a second state; selecting a set of the MEM devices located at an intersection of at least one row of the array and the column, with the set of MEM devices being in the second state; switching all the MEM devices in the column of the array, except for the set of the MEM devices, from the second state to the first state; and repeating the above steps for each column of the array. The method of the present invention allows latching of particular MEM devices in the second state until all electrical power is removed from the MEM array.
The step for switching all of the MEM devices in the column of the array from the first state to the second state can comprise applying an actuation voltage to all of the MEM devices in the column of the array for electrostatically switching the MEM devices from the first state to the second state. The step for selecting the set of the MEM devices can comprise applying a holding voltage to all of the MEM devices in one or more rows of the array, with the holding voltage being of insufficient magnitude to switch any of the MEM devices in the rows from the first state to the second state, but being of sufficient magnitude to maintain the set of MEM devices in the second state after removal of the actuation voltage (i.e. the holding voltage latches the MEM devices in whichever state they were already in when the holding voltage is applied). The step for switching all the MEM devices in the column of the array, except for the set of the MEM devices, from the second state to the first state can comprise the steps of removing the actuation voltage from all the MEM devices in the column of the array; applying a maintaining voltage to all the MEM devices in the column of the array: and removing the holding voltage from all the MEM devices in the row of the array. The maintaining voltage can be either equal in magnitude with the holding voltage or can be different in magnitude from the holding voltage.
Applying the actuation voltage to all of the MEM devices in the column of the array can be performed by applying the actuation voltage to a first electrode underlying a moveable member of each MEM device in the column of the array, while applying the holding voltage to all of the MEM devices in the row of the array can be performed by applying the holding voltage to a second electrode underlying the moveable member of each MEM device in the row of the array. The maintaining voltage can be applied to the first electrode or to a third electrode underlying the moveable member of each MEM device in the column of the array depending upon a structure of the MEM device used with the method of the present invention.
The method of the present invention can further comprise a step for sensing whether one of the MEM devices in the array is in the first state or in the second state at an instant in time. The sensing step can be performed either capacitively (e.g. by using the capacitance between the moveable member and a sensing electrode underlying or overlying the moveable member) or optically (e.g. by providing a light beam incident on a surface of the moveable member and sensing the angular position or phase of a reflected component of the incident light beam).
The present invention also relates to a method for electrically addressing an array of MEM devices, comprising steps for applying an actuation voltage to all of the MEM devices in a column of the array, thereby electrostatically actuating all of the MEM devices in the column; applying a holding voltage to all of the MEM devices in at least one row of the array, thereby selecting the MEM devices located at an intersection of the row and the column, with the holding voltage being of insufficient magnitude to electrostatically actuate any of the MEM devices in the row, but being of sufficient magnitude to maintain the actuation of the MEM devices located at the intersection of the row and the column when the actuation voltage to the column is removed; removing the actuation voltage from the column, and applying a maintaining voltage to the column; removing the holding voltage from the row; and repeating each of the steps listed above for each column in the array.
The step for applying the actuation voltage to all of the MEM devices in the column of the array can comprise applying the actuation voltage to a first electrode underlying a moveable member of each MEM device in the column of the array to electrostatically change a position of the moveable member from a first state to a second state. The step for applying the holding voltage to all of the MEM devices in the row of the array can comprise applying the holding voltage to a second electrode underlying the moveable member of each MEM device in the row of the array.
The step for removing the actuation voltage from the column and applying the maintaining voltage to the column can comprise removing the actuation voltage from the first electrode and applying the maintaining voltage to the first electrode. Alternately the maintaining voltage can be applied to a third electrode underlying the moveable member of each MEM device in the column of the array. The maintaining voltage can be equal in magnitude to the holding voltage or different therefrom depending upon a particular structure of the MEM devices in the array.
The method of the present invention can further include a step for sensing the position of the moveable member of one or more MEM devices in the array for determining the state of the MEM devices at a particular time. Sensing the position of the moveable member in the MEM devices can be performed by either capacitively sensing the position or optically sensing the position.
The definition of the first and second states will in general depend upon the exact structure of the MEM devices and the extent to which the moveable member can be switched in position or angle. As an example, in certain embodiments of the present invention, the first state can be defined by the moveable member being coplanar with a substrate whereon the array is formed; and the second state can be defined by the moveable member being tilted at an angle to the substrate. In other embodiments of the present invention, the first state can be defined by the moveable member being located in an as-formed position; and the second state can be defined by the moveable member being displaced downward from the as-formed position. In yet other embodiments of the present invention, the first state can be defined by the moveable member being oriented at an angle to a substrate whereon the array is formed; and the second state can be defined by the moveable member being oriented at a different angle with respect to the substrate. The present invention is applicable to arrays of MEM devices in the form of micromirrors, memory elements or both.
The present invention further relates to a method for electrically addressing an array of MEM devices formed on a substrate, comprising steps for applying an actuation voltage to all of the MEM devices in a column of the array, thereby electrostatically actuating all of the MEM devices in the column to change the position of a moveable member of each MEM device from a first state to a second state; selecting a set of the MEM devices in the column that will remain in the second state when a maintaining voltage having a magnitude less than the actuation voltage will be later substituted for the actuation voltage; and repeating the above two steps for each column in the array. The step for selecting the set of MEM devices further comprises applying a holding voltage to one or more rows of the array while the actuation voltage is applied to the column, thereby selecting the MEM devices having both the actuation voltage and the holding voltage applied thereto for the set of MEM devices, with the holding voltage being of insufficient magnitude to electrostatically actuate any of the MEM devices in the column, but being of sufficient magnitude to maintain any MEM device in the column to which the holding voltage is applied in the second state when the actuation voltage is no longer present; substituting the maintaining voltage for the actuation voltage while retaining the holding voltage in place; and removing the holding voltage. Each MEM device in the array can comprise, for example, a micromirror or a memory element or both.
Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.